I grow lots of Geraniums in pots. Mine get time release fertilizer when I plant them and nothing else except a bit of water soluable 15-30-15 near the end of summer when the time release is done. Before I started using time release fertilizer I used the water soluable every 3rd watering.

Whatever you use it should be a balanced fertilizer. Too much nitrogen and you'll get nothing but leaves

Thanks for the tip about avoiding too much nitrogen.

Does the brand of time release matter? Which brand did you use?

I have used Smartcote and CIL time release. I used Smart cote the year before last and I found a good deal on CIL last year, so I stocked up. I couldn't see any difference between the years, so I don't think it really matters. This year I used CIL again. They are all a balanced fertilizer, with all fairly equal numbers.

Many thanks for the names. I searched for both and couldn't find a US source. I'm going to try ebay next.